Hydrocyanic acid (or “hydrogen cyandide”) (HCN) can be commercially produced by reacting ammonia with natural gas (methane) and an oxygen-containing gas at an elevated temperature in a converter in the presence of a suitable catalyst (U.S. Pat. No. 1,934,838). The exit gas from the converter contains unreacted ammonia together with small amounts of oxalic acid and formic acid (or precursors of oxalic acid and formic acid). The unreacted ammonia is separated from HCN in an absorber by contacting the converter exit gas with an aqueous solution of ammonium phosphate in the ammonia absorber. The initial ammonium to phosphate molar ratio in the ammonium phosphate solution is in the range of 1 to 1.5, and this solution can be referred to as “lean ammonium phosphate solution.” The oxalic acid and formic acid end up in the aqueous solution, where they are converted to oxalates and formates. After absorption, the final ammonium to phosphate molar ratio in the ammonium phosphate solution is in the range of 1.5 to 2.0, and this solution can be referred to as “rich ammonium phosphate solution.” The rich ammonium phosphate solution is converted back to lean ammonium phosphate solution by stripping ammonia with water vapor in a distillation column (ammonia stripper). The lean ammonium phosphate solution thus obtained is reused in the ammonia absorber. The distillation operation in the ammonia stripper does not remove the oxalates and formates. As a result, there is accumulation of these compounds in the ammonium phosphate solution. Presence of the oxalate and the formate compounds can cause corrosion and failure of process equipment. A concentration of oxalate greater than 1% and a concentration of formate greater than 3% are undesirable. Therefore, a part of the ammonium phosphate solution must be continuously purged from the loop as waste. A waste treatment cost is associated with this purge. The process also requires addition of make-up ammonium phosphate solution, with an associated cost.
It is known that the freezing point of ammonium phosphate solution decreases with increases in its concentration. In the process described above, the ammonia absorber is operated at a temperature of about 55 degrees C. The presence of oxalates and formates in the lean ammonium phosphate solution help to raise the freezing point of the solution. However, even with oxalates and formates present, the upper limit on the concentration of ammonium phosphate solution used is about 35% in order to avoid freezing of ammonium phosphate solution and resulting equipment plugging caused by the frozen solution. If, however, oxalates and formates could be eliminated from the lean ammonium phosphate solution, the freezing point would be depressed, with the effect that higher concentrations of ammonium phosphate could be employed without the risk of freezing. Such higher concentrations of ammonium phosphate should result in greater process capacity.
It would be desirable, therefore, to have a process for making HCN in which oxalate and formate compounds are continuously decomposed, avoiding build up in the process with a need for substantial purges, and allowing for the use of higher lean ammonium phosphate concentrations with increased process capacity.